Polyimide foams and their preparation

ABSTRACT

Described are flexible polyimide foams having enhanced compression fatigue life and softness for use in the manufacture of seat cushions and methods for the production of such foams and precursors therefor. These foams are produced from novel polyimides prepared by reaction of an organic tetracarboxylic acid or derivative thereof, preferably an ester with (a) about 1 to about 50 mol percent of a diester of (i) a primary amino-substituted aromatic carboxylic acid, and (ii) a polymethylene glycol, and (b) at least one aromatic or heterocyclic primary diamine. Foams can be produced having (a) a fatigue life as determined by ASTM test procedure D 3574-81 using foam specimens from three to five inches in thickness of at least 15,000 cycles, or (b) an indentation force deflection as determined by ASTM test procedure D3574-81 on foam specimens of one-inch thickness of less than 40 pounds of force at 25% deflection and less than 180 pounds of force at 65% deflection, or both of (a) and (b).

This application is a division of application Ser. No. 187,493, filedApr. 28, 1988.

TECHNICAL FIELD

This invention relates to new and useful polyimides, and moreparticularly to novel polyimides and polyimide foams having improvedproperties, to novel precursors from which such polyimides and polyimidefoams can be prepared, and to processes for their preparation.

BACKGROUND

A great deal of effort has been devoted to the development of polyimidesand polyimide foams having useful properties. One promising area ofapplication for flexible polyimide foams is as a potential substitutefor flammable polyurethane foams in aircraft seat cushions and the like,where the fire resistance and lightness of polyimides would be ofconsiderable advantage.

Unfortunately, previously known flexible polyimide foams generallysuffer from either or both of two shortcomings limiting their usefulnessfor seat cushion applications:

1. Lack of sufficient compression fatigue life, which means that thefoam prematurely loses its resiliency (ability to return to its originalshape) after repeated compression during usage.

2. Lack of sufficient softness which causes the cushion to be stifferand less comfortable than desired.

THE INVENTION

This invention provides new and useful flexible polyimides which canovercome either or both of the foregoing shortcomings.

Pursuant to one embodiment there is provided a polyimide foam preparedby reaction of an organic tetracarboxylic acid or derivative thereof(e.g., salt, acid halide, anhydride or preferably ester thereof) with(a) about 1 to about 50 mol percent of a diester of (i) a primaryamino-substituted aromatic carboxylic acid and (ii) a polymethyleneglycol, and (b) at least one aromatic or heterocyclic primary diamine.These polyimides per se constitute an additional embodiment of thisinvention.

Another embodiment of this invention involves provision of a foamablepolyimide precursor comprising an essentially stoichiometric mixture of(a) at least one organic tetracarboxylic acid ester, and (b) a mixtureof at least two primary diamines, one such diamine being about 1 toabout 50 mol percent of a diester of (i) an amino-substituted aromaticcarboxylic acid and (ii) a polymethylene glycol, and a second suchdiamine being an aromatic or heterocyclic diamine.

A still further embodiment of this invention involves a method ofpreparing a polyimide foam which comprises reacting an essentiallystoichiometric mixture of (a) at least one organic tetracarboxylic acid(or derivative thereof, preferably an ester), and (b) at least twoprimary diamines, one such diamine being about 1 to about 50 mol percentof a diester of (i) an amino-substituted aromatic carboxylic acid and(ii) a polymethylene glycol, and a second such diamine being an aromaticor heterocyclic diamine; and heating the reaction mixture to cure itinto polyimide foam. When using the free tetracarboxylic acid or a salt,acid halide or anhydride thereof, a suitable blowing agent should bepresent in the reaction mixture to cause the foam structure to bedeveloped. Use of the ester is preferred as this results in thedevelopment of the foam structure even without use of a blowing agent.

Yet another embodiment of this invention is a polyimide foam having (a)a fatigue life as determined by ASTM test procedure D 3574-81 using foamspecimens from three to five inches in thickness of at least 15,000cycles, or (b) an indentation force deflection as determined by ASTMtest procedure D3574-81 on foam specimens of one-inch thickness of lessthan 40 pounds of force at 25% deflection and less than 180 pounds offorce at 65% deflection, or both of (a) and (b). For the purposes ofthis invention, failure in the foregoing fatigue life test procedure iseither (i) a thickness loss of more than 10%, (ii) a loss in indentationforce deflection at 40% deflection of more than 10%, or (iii) asignificant visually-perceivable surface cracking.

The above and other embodiments, features and advantages of thisinvention will become still further apparent from the ensuingdescription and appended claims.

In the practice of this invention the flexible polyimides are formed byuse of a combination of primary diamines, one of which is a diester ofan amino-substituted aromatic carboxylic acid and a polymethyleneglycol, and a second of which is a different aromatic diamine or aheterocyclic diamine. Such diesters may be represented by the generalformula:

    H.sub. 2N--ArCOO--R--OOCAr--NH.sub. 2

wherein R is an alkylene group (which may be branched or straight chain)and which preferably contains from 3 to 8 carbon atoms, most preferablytrimethylene; and Ar is an aromatic group which may be composed of oneor more fused or non-fused benzene rings which in turn may carrysuitable substituents (e.g., nitro, alkoxy, etc.) in addition to theprimary amino groups.

A few exemplary diesters of this type include:

ethylene glycol-4-aminobenzoic acid diester;

ethylene glycol-3-aminobenzoic acid diester;

ethylene glycol-2-aminobenzoic acid diester;

trimethylene glycol-3-aminobenzoic acid diester;

trimethylene glycol-2-aminobenzoic acid diester;

trimethylene glycol-3-amino-2-nitrobenzoic acid diester;

tetramethylene glycol-3-amino-4-nitrobenzoic acid diester;

tetramethylene glycol-3-amino-5-nitrobenzoic acid diester;

tetramethylene glycol-4-amino-2-nitrobenzoic acid diester;

1,5-pentanediol-4-amino-3-nitrobenzoic acid diester;

1,6-hexanediol-5-amino-2-nitrobenzoic acid diester;

neopentyl glycol-4-amino-2-methylbenzoic acid diester;

1,8-octanediol-4-amino-2-propylbenzoic acid diester;

1,9-nonanediol-3-amino-4-methylbenzoic acid diester;

1,10-decanediol-4-(4-aminophenyl)benzoic acid diester;

and the like. Mixture of such diesters may be employed.

A particularly preferred diester of this type is the diester oftrimethylene glycol (1,3-propanediol) and 4-aminobenzoic acid.

The other organic diamines with which the foregoing diamino-substituteddiesters are employed may be represented by the formula:

    H.sub.2 N--R'--NH.sub.2

wherein R' is an aromatic group containing 5 to 16 carbon atoms andcontaining up to one hetero atom in the ring, the hetero atom beingnitrogen, oxygen or sulfur. Also included are aromatic groups such as:##STR1## Representatives of such diamines include: 2,6-diaminopyridine;

3,5-diaminopyridine;

3,3'-diaminodiphenylsulfone;

4,4'-diaminodiphenylsulfone;

4,4'-diaminodiphenylsulfide;

3,3'-diaminodiphenylether;

4,4'-diaminodiphenylether;

meta-phenylenediamine;

para-phenylenediamine;

4,4'-methylene dianiline;

2,6-diamino toluene;

2,4-diamino toluene;

and the like.

It is also possible and sometimes desirable in the preparation of thepolyimide precursors of this invention, to include in the reactionmixture one or more aliphatic diamines. Such aliphatic diamines arepreferably alpha-omega diaminoalkanes having the formula:

    H.sub.2 N--(CH.sub.2).sub.n --NH.sub.2

(I)

wherein n is an integer from 2 to 16. Representatives of such diaminesinclude 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane,1,6-diaminohexane, etc.

In place of or in addition to the foregoing aliphatic amines, use can bemade of aliphatic etherified polyamines of the type polyoxypropyleneamines having the formula:

    H.sub.2 N--CH(CH.sub.3)CH.sub.2 --[OCH.sub.2 CH(CH.sub.3)].sub.x --NH.sub.2 (II)

wherein x varies from 1 to about 5 carbon atoms.

Other useful primary diamines which may be included in the products ofthis invention include amine-terminated butadiene-nitrile copolymershaving the general formula: ##STR2## wherein R is either a phenylenegroup or an alkylene group, R₁ is hydrogen or methyl, and x and y oreach independently integers ranging from 1 to 25 and n is an integer,preferably below 20. In these copolymers it is preferred that butadieneconstitute at lesat 50% by weight of the butadiene and nitrile monomer.The nitrile monomer copolymerized with the butadiene can either beacrylonitrile or methacrylonitrile. Such copolymers generally have lowmolecular weights, preferably less than 3,000 to insure that they aresufficiently fluid to react in the formation of the polyimide as well assufficiently fluid so as to be capable of foaming.

Still another type of primary diamines which may be included in theproducts of this invention is the aromatic amino-terminated silicones,such as those having the general formula: ##STR3## wherein R is a C₂ toC₆ alkylene group, R₁ and R₂ are each independently lower alkylcontaining 1 to 3 carbon atoms and n is an integer from 1 to 4.

In the practice of this invention the organic tetracarboxylic acidpreferably in the form of its diester, most preferably from methanol orethanol, is reacted with the above-referred-to combination of amines toform a prepolymer in the form of a consolidated, fragile foam structure,which is then subjected to additional heating in order to effect imideformation and thereby cure the polymer. When using the tetracarboxylicacid ester this operation can be conducted either in the presence orabsence of an added blowing agent to provide the desired polyimide foam.

The tetracarboxylic acid esters preferably employed in the practice ofthis invention have the general formula: ##STR4## wherein A is atetravalent organic group and R₁ to R₄ inclusive are independentlyhydrogen or lower alkyl, most preferably methyl, ethyl, or propyl, Thetetravalent organic group A is preferably one having one of thefollowing structures: ##STR5## wherein X is one or more of thefollowing: ##STR6## Preferred among the tetracarboxylic acid esters arethe alkyl esters of 3,3',4,4'-benzophenone tetracarboxylic acid, mostpreferably the lower alkyl diesters thereof. Mixtures of two or morearomatic esters, most preferably predominating in diesters, may beemployed, if desired.

It is also possible, in accordance with this invention, that thetetracarboxylic acid derivative employed in the manufacture of thepolyimide foams be a caprolactam as taught by U.S. Pat. Nos. 4,161,477,4,183,838 and 4,183,839, the disclosures of which are incorporatedherein by reference. As described in those patents, a bis-imide isformed by reaction of a tetracarboxylic acid dianhydride with anoxoimine such as caprolactam and then reacted with the diamine ordiamines to produce the desired polyimides. The caprolactam is displacedduring the reaction, in much the same way as is the ester portion of thetetracarboxylic acid ester.

The relative proportions used in the preparation of the precursors andpolymers of this invention can be varied. In general, it is preferred toemploy essentially stoichiometric proportions as between thetetracarboxylic acid ester and the combination of primary diamines.However, non-stoichiometric mixtures can be used although the excess ofthe reactant present in excess usually does not participate in thereaction. As noted above, about 1 to about 50 mol percent (preferablyabout 20 to about 50 mol percent) of the combination of primary diaminesemployed is composed of one or more diesters between anamino-substituted aromatic carboxylic acid and a polymethylene glycol.The balance of the combination is composed of aromatic or heterocylicdiamine(s) with or without the addition of still other diamines, forexample diamines of the type referred to hereinabove in formulas I, II,III and IV, or any mixture thereof. Usually the overall combination ofamines will contain no more than about 10 mol percent of these otherdiamines.

In accordance with one preferred form of the invention, use is made of acombination of aromatic amines, one of which is a nitrogen heterocyclicdiamine, preferably 2,6-diaminopyridine and/or 3,5-diaminopyridine,while the other is a diamine containing two benzene rings, preferably4,4'-methylenedianiline and/or 4,4'-oxydianiline. When using acombination of aromatic amines in accordance with this concept, the molratio of the non-heterocyclic diamine to the nitrogen-containingheterocyclic diamine is within the range of 1.0 to 3.0, and preferably1.5 to 2.8.

When using a lower alkyl ester of the tetracarboxylic acid, theresulting alcohol produced in the reaction as well as the water releasedduring the reaction can be used as the blowing agent duringpolymerization to form the desired polyimide foams. Alternatively, usecan be made of any of a variety of organic or inorganic blowing agents.By use of a solid blowing agent such as Celogen TSH, Celogen OT, CelogenAZ 130, Celogen RA, Celogen HT 500, Celogen HT 550, sodium bicarbonate,benzenesulfonyl hydrazide, boric acid, benzoic acid, and Expandex 5 PTof controlled particle size, the homogeneity of the cellular structureof the resulting polyimide foam can be more accurately controlled.Preferred for such use are solid blowing agents which have beensubjected to ball milling or other grinding so that the blowing agent isless than 200 microns in diameter, with 98 percent of the blowing agentparticle size being less than 150 microns in diameter.

The chemical compositions of the blowing agents identified by trade nameabove follows:

    ______________________________________                                        Blowing Agent Chemical Composition                                            ______________________________________                                        Celogen TSH   toluenesulfonyl hydrazide                                       Celogen OT    p,p'-oxybis(benzenesulfonyl hydrazide                           Celogen AZ 130                                                                              azodicarbonamide                                                Celogen RA    p-toluenesulfonyl semicarbazide                                 Celogen HT 500                                                                              a modified hydrazine derivative                                 Celogen HT 550                                                                              hydrazol dicarboxylate                                          Expandex 5 PT 5-phenyltetrazole                                               ______________________________________                                    

Variations in the concentration of the blowing agent can be used toachieve specific densities and ILD values. Concentrations of up to 10percent based on the weight of the polyimide precursor, and preferably 1to 5 percent, can be employed. A concentration of about 2.5 weightpercent is particularly preferred.

Hydrated organic compounds of the type referred to in U.S. Pat. No.4,621,015 may also be used as blowing agents in the process.

In the practice of this invention, it is possible to include in thereaction mixture various filler and/or reinforcing materials. Forexample, graphite, glass and other synthetic fibers can be added to thecomposition to produce a fiber-reinforced product. Microballons may beadded for density adjustment, if desired. It is frequently desirable toemploy a surfactant thereby increasing cellular structure stability anduniformity, and increase fatigue resistance and make the foam moreflexible and resilient. The nature of such surfactants for this use iswell known and reported in the patent literature.

Although not necessary, for some applications it is desirable to add asuitable quantity of a flame retardant material to the formulation inorder to still further increase the flame resistance of the resultantfoam.

In preparing the precursors of this invention, it is preferred to employthe procedures and spray drying techniques described in U.S. Pat. No.4,296,208, the disclosure of which is incorporated herein by reference.

The temperatures at which the precursor is converted to the polyimidefoam are generally those temperatures used in the preparation of otherpolyimide polymers. As a general rule temperatures ranging from 200° to400° C. can be used, with heating times from 5 to 60 minutes or longer.As those skilled in the art will appreciate, the time for carrying outthe reaction is somewhat dependent upon the reaction temperature, highertemperatures enabling the use of shorter reaction times. It is alsopossible to heat to a lower temperature in the first stage of thereaction and then to higher temperatures in the later stages.

Heating can be carried out in a conventional oven if desired.Alternatively, the foaming and curing of the precursor into a foamedpolyimide polymer can be effected by means of microwave heating. In thistechnique, the precursor is exposed for 1 to 120 minutes to radiofrequencies within the range of 915 to 2450 MHz, with the power outputranging from 1 to 100 kw. The power output to prepolymer weight ratiogenerally falls within the range of 0.1 to 10 kw per kg.

Having described the basic concepts of the invention, reference is nowmade to the following examples which are provided by way ofillustration, but not by way of limitation of the practice of theinvention.

The following abbreviations are used in the examples:

MDA --4,4'-Methylenebisaniline

DAP --2,6-Diaminopyridine

TGD --Trimethylene glycol di-p-aminobenzoate

ATBN --Amino-terminated poly(butadiene-acrylonitrile), (HYCAR 1300X16)

ODA --4,4'-Oxydianiline

BTDA --Benzophenone tetracarboxylic acid dianhydride

BTDE --Benzophenone tetracarboxylic acid, methyl ester

DC-193 --Polysiloxane surfactant (Dow Corning Corporation)

pcf --Pounds per cubic foot

IFD --Indentation force deflection as measured by the Indentation ForceTest of ASTM Test Designation D 3574-81

All polyimide foams were produced using a two-stage microwave-thermaloven procedure. The microwave was a Gerling Moore 5.5 kw microwavecavity having two microwave sources, only one of which was used. Lowerpower (one source) was 1.5 kw and full power (one source) was 2.75 kw.Roller fatigue results were measured using the Dynamic Fatigue Test bythe "Roller Shear at Constant Force" according to ASTM Test DesignationD 3574-81.

EXAMPLE 1 Formulation

[(0.47 mol MDA+0.3 mol DAP+0.25 mol TGD) per mol BTDA]+1.86% DC-193based on the combined weights of the monomers.

Procedure

A 5-liter, 3-necked round bottom, glass flask in a heating mantleequipped with a stirrer, reflux condenser and thermometer was chargedwith 1612 g (5 mols) of BTDA, 1286 g (40.14 mols) of methanol, and 60 gof water. The heater and stirrer were switched on. The solution wasmilky off-white in appearance. After 30 minutes with the temperature at50° C., the heater was switched off and the mixture was stirred foranother 19 minutes. At this point the temperature had reached 71° C. andthe reaction mixture had turned into a clear, deep amber solutionindicating completion of the esterification reaction. While stirring thesystem and keeping the temperature between 50 and 65° C., the diamineswere added using methanol dilution and washes, in the following amountsand sequence: 393 g (1.25 mols) TGD; 446 g (2.35 mols) MDA; 164 g (1.50mols) DAP. A total of 805 g of methanol was added during theseoperations which occurred over a period of about 2 hours. Next, with thetemperature of the system at 44° C., 49 g of DC-193 diluted withmethanol was added, again using a methanol rinse and the system wasstirred for another 2 hours and 15 minutes.

The reaction solution was spray dried under nitrogen in a Niro MobileMinor srapy dryer. In this operation the solution was fed to the dryerover a 2-hour period with the inlet temperature between 100 and 110° C.,the outlet temperature between 68.4 and 70.1° C., and the atomizer speedbetween 28,000 and 34,900 rpm. This yielded 3,018 g of polyimideprecursor in powder form. The powder was sifted through a sieve with 425micron openings and kept in a sealed plastic bag until used.

Using separate portions of precursor, three polyimide foams wereproduced. In each case, a free-rise foaming procedure was used (i.e., nomold was utilized). In one run the foam was produced by microwaving for5 minutes at 1.5 kw and for 10 minutes at 2.75 kw followed by use of athermal oven held at 490° F. for 1 hour and 15 minutes. In another runthe sample was treated in the microwave for 10 minutes at 2.75 kwfollowed by exposure to 490° F. in a thermal oven for 1 hour and 3minutes. The third sample was produced in the same fashion as the secondsample except that the time in the thermal oven was 1 hour and 15minutes. Using trimmed sections from the resultant foams, measurementsand observations were made of their properties.

Results

The foams were of very good quality with relatively fine cellstructures. They were extremely soft, resilient, flexible andnon-brittle. The average density was 0.49 pcf.

The IFD (1 inch thick specimen) was 17 pounds of force at 25% deflectionand 66 pounds of force at 65% deflection. The roller fatigue test (3inch thick specimen) was terminated at 14,000 cycles with little surfacedamage and 15% average thickness loss. The foam had a tensile strengthof 11 psi and an elongation at break of 28%.

EXAMPLE 2 Formulation

[(0.53 mol MDA+0.3 mol DAP+0.2 mol TGD) per mol BTDA]+1.86% DC-193 basedon the combined weights of the monomers.

Procedure

BTDE was prepared in a 2-liter, 3-necked flask using 386.7 g (1.20 mols)of BTDA, 307.6 g (9.60 mols) of methanol and 14.5 g of water during a 45minute reaction period with a temperature controlled between 22 and 70°C. The diamines were then added to the deep, clear amber solution. Thefollowing sequence and amounts of addition were used: 127.1 g (0.64 mol)of MDA; 39.2 g (0.36 mol) of DAP; 75.2 g (0.24 mol) of TGD. Theseadditions occurred over a period of about 38 minutes with thetemperature between 45 and 51° C. Methanol rinses were employed.Thereupon the heater was turned on and the mixture stirred for 23minutes during which time the temperature increased from about 40° C. toabout 62° C. Next, 11.7 g of DC-193 dissolved in methanol was added,again using a methanol rinse. The resultant reaction solution was thendried using a vacuum oven with the vacuum adjusted from 25 to 29 inchesof mercury and a temperature of 150° F. The powder was sieved andconverted into a polyimide foam using a polypropylene mold in a 2-stagemicrowave-thermal oven procedure. The powder was subjected tomicrowaving for 20 minutes at 1.5 kw. In the thermal oven thetemperatures were 455° F. for 10 minutes, 470° F. for 22 minutes and500° F. for 56 minutes. Trimmed sections of the foam were used indetermining the physical properties described below.

Results

The polyimide foam had a density of 0.58 pcf, a tensile strength of 10psi and an elongation of 64% at break.

EXAMPLE 3 Formulation

[(0.34 mol MDA+0.34 mol DAP+0.34 mol TGD) per mol BTDA]+1.87% DC-193based on the combined weights of the monomers.

Procedure

BTDE was produced at 23°-70° C. from 387 g (1.20 mols) of BTDA, 308 g(9.61 mols) of methanol and 15 g of water. To the resultant clear, darkamber solution were added TGD (129 g; 0.41 mol), MDA (81 g; 0.41 mol)and DAP (45 g; 0.41 mol). The additions were facilitated by use ofmethanol rinses and the temperature of the reaction mixture wascontrolled between 36 and 64° C. over a period of 1 hour 48 minutes. Tothe reaction mixture was then added 12.0 g of DC-193 using methanoldilution and rinse. The resultant mixture was stirred for about 1.5hours.

The polyimide precursor was isolated in powder form by use of a vacuumoven operated generally as in Example 2. The yield of dried polyimideprecursor was 731 g. Sieved polyimide precursor was converted intopolyimide foam in a mold by use of the 2-stage microwave-thermal ovenprocedure. The microwave portion of the cycle involved 20 minutes at 1.5kw. The final curing in the oven occurred at 470° F. over a period of1.5 hours.

Results

The polyimide foam had a density of 0.56 pcf.

EXAMPLE 4 Formulation

[(0.72 mol MDA+0.30 mol TGD) per mol BTDA]+1.97% DC-193 based on thecombined weights of the monomers.

Procedure

A methanol solution of BTDE was produced from 818 g (2.5 mols) of BTDA,641 g (20.01 mols) of methanol and 31 g of water. The reactiontemperature was raised from 25° to 72° C. during a reaction period ofabout 1 hour. To this solution were added the following ingredients inthe following sequence: TGD (245 g; 0.76 mol); MDA (355 g; 1.79 mols)and 28 g of DC-193. These ingredients were added as methanol solutionsand methanol rinses were employed. The maximum reaction temperature was61° C. The polyimide precursor was recovered in powdered form by use ofa spray drying procedure generally as in Example 1 using an inlettemperature between 96 and 102° C., an outlet temperature between 68.8and 69.7° C. and an atomizer speed ranging from 31,600 to 32,800 rpm.This resulted in a recovery of 1,515 g of polyimide precursor which wasstored in a jug until use. Polyimide foams were produced using the2-stage free-rise microwave-thermal oven procedure (microwave: 2.75 kwfor 15 minutes; thermal oven: 480° F. for 1 hour 32 minutes). Trimmedsections from the resultant foams were used for physical propertydeterminations and observations.

Results

The foam was soft, extremely flexible and resilient at room temperature,with a non-homogeneous cell structure. It had a density of 0.92 pcf. Theroller fatigue test (31/8 inch thick specimen) was terminated after31,866 cycles with an average thickness loss of 16%, a weight loss of1.1% and some severe cracking. At 14,725 cycles, virtually no damage tothe foam was observed in the roller fatigue test.

EXAMPLE 5 Formulation

[(0.31 mol TGD+0.71 mol MDA+0.00038 mol ATBN) per mol BTDA]+2.35% DC-193based on the combined weights of the monomers.

Procedure

The following ingredients were used to produce BTDE: 981 g (3.00 mols)of BTDA; 769 g (23.76 mols) of methanol; and 36 g of distilled water. Tothe methanol solution of BTDE was added 1.98 g of ATBN (HYCAR 1300X16,B. F. Goodrich Chemical Company) with the temperature of the methanolsolution at 65° C. Heat was then applied and solution brought to refluxtemperature for one hour. Then, the other diamines were added by use ofmethanol dilution and rinses in the order of TGD (294 g; 0.92 mol) andMDA (425 g; 2.14 mols). A total of 509 g of methanol was used in theseadditions. During the additions the temperatures were maintained between50 and 63° C. Finally, 40 g of DC-193 was added together with dilutionmethanol and the solution stirred for an additional 15 minutes. Thepolyimide precursor was recovered in powder form by use of a spray dryeroperated generally as in Examples 1 and 4. The recovery was 1,878 g. Oneportion of the precursor was placed in a 260° C. air oven for 30 minutesand the resultant foam was subjected to a T_(g) determined by DSCanalysis. Polyimide foam was produced in a mold from the polyimideprecursor powder by use of the 2-stage microwave-thermal oven procedure(microwave: 2.75 kw for 20 minutes; thermal oven: 470° F. for 33 minutesand 480° F. for 1 hour and 11 minutes). Trimmed sections of the foamswere subjected to physical property determinations.

Results

The foam had a T_(g) of 242° C., a density of 0.92 pcf, and an IFD (1inch specimen) of 25 pounds of force at 25% deflection and 112 pounds offorce at 65% deflection. The roller fatigue test (35/8 inch thickspecimen) was terminated at 21,972 cycles with no weight loss and anaverage thickness loss of 2%.

EXAMPLE 6 Formulation

[(0.31 mol TGD+0.71 mol MDA) per mole BTDA]+1.94% DC-193 based on thecombined weights of the monomer. Samples were also produced containingeither zinc borate or alumina trihydrate fire retardants.

Procedure

A methanol solution of BTDE was produced from 1,636 g (5.00 mols) ofBTDA, 1,283 g (39.64 mols) of methanol and 60 g of water using thegeneral procedure of Example 1. To this solution were added 490 g (1.53mols) of TGD and 709 g (3.57 mols) of MDA. A total of 847 g of dilutionmethanol was used for these additions. Temperatures were controlledduring the additions to between 49 and 60° C. Then, 55 g of DC-193 wasadded as a methanol solution. The product was spray dried and theresultant dry powder sifted through a No. 25 screen. Polyimide foam wasproduced from the polyimide precursor (microwave: 10 minutes at 2.75 kw;thermal oven: 1 hour at 475° F.). Into four additional individualquantities of the powdered polyimide precursor were mixed various flameretardants, as follows: Foam A--18 g zinc borate powder (Firebrake ZB;U. S. Borax & Chemical Company) per 100 g polyimide precursor; FoamB--18 g alumina trihydrate powder (SB-632; Solem Industries) per 100 gpolyimide precursor; Foam C--18 g alumina trihydrate powder (Akrochem8.0; Akron Chemical Company) per 100 g polyimide precursor, and FoamD--25 g alumina trihydrate powder (SB-136; Solem Industries) per 100 gpolyimide precursor. Each of these mixtures was converted into apolyimide foam by the 2-stage microwave-thermal oven procedure using thefollowing conditions: Foam A--microwave: 1.5 kw for 10 minutes; thermaloven: 475° F. for 63 minutes; Foam B--same as Foam A except 65 minutesin the thermal oven; Foam C--microwave: 1.5 kw for 10 minutes; thermaloven: 470° F. for 43 minutes and 480° F. for 27 minutes; and FoamD--microwave: 1.5 kw for 15 minutes; thermal oven: 475° F. for 1 hourand 9 minutes. All foaming operations in this Example were conductedwithout use of a mold.

Results

Each of the foams was soft, flexible and resilient. The baseline foam(i.e., without added flame retardant) had a density of 1.12 pcf andshowed an LOI (ASTM D 2863-77) of 32-33. Foam A had a density of 0.71pcf and exhibited an LOI of 41-42. Foam B had a density of 0.77 pcf andexhibited an LOI of 38-39. The density of Foam C was 0.73 pcf with anLOI of 37-38. Foam D had a density of 0.67 of pcf and exhibited an LOIof 39-40.

EXAMPLE 7 Formulation

[(0.31 mol TGD+0.71 mol ODA) per mole of BTDA]+1.94% DC-193 based on thecombined weights of the monomers.

Procedure

A solution of BTDE in methanol was produced using BTDA (981 g; 3.00mols), methanol (768 g; 23.73 mols) and distilled water (36 g). Thediamines were added as follows: TGD (294 g; 0.92 mol); and ODA (430 g;2.14 mols) using methanol dilution and washes. During these additionsthe temperature was controlled between 45 and 68° C. Then, 33 g ofDC-193 diluted with methanol was added. During these operations of totalof 510 g of methanol was added to the system. The reaction mixture wasstirred for about 3 hours. The polyimide precursor was recovered by useof a spray dryer and sieved through a No. 25 screen. Using a mold, thepolyimide precursor was converted into polyimide foam by exposure for 15minutes in a microwave (2.75 kw power) and 1.5 hours to 475° F. in athermal oven.

Results

The foam had a density of 0.92 pcf. The roller fatigue test using a foamspecimen of 4.5 inches in thickness was terminated at 40,000 cycles withvery minor damage to the surface, a weight loss of 0.7% and an averagethickness loss of 3.5%. An 8.3% loss in IFD at 40% deflection wasincurred.

EXAMPLE 8 Formulation

[(0.31 mole TGD+0.71 mol MDA) per mole BTDA]. Samples were also producedcontaining various surfactants and flame retardants.

Procedure

A methanol solution of BTDE was formed from 1,636 g (5.00 mols) of BTDA,1,282 g (39.61 mols) of methanol and 60 g of water. Diamines added tothe system were TGD (490 g; 1.53 mols) and MDA (709 g; 3.57 mols). As inthe above examples, methanol rinses and dilution were employed. To aidin the dissolution of the TGD the temperature was raised to 70° C. Theresin solution was cooled to 40°-45° C. and subdivided into weighedportions by pouring into 1,000 mL Erlenmeyer flasks. The followingadditives were measured into the respective resin solutions:

Solution A: 6.12 g DC-193 and 16.2 g dimethyl methylphosphonate in 484 gof resin solution

Solution B: 6.00 g DC-193 and 15.55 g Antiblaze 1045 phosphate ester(Albright & Wilson Inc.) in 503 g of resin solution

Solution C: 6.45 g DC-193 in 485 g of resin solution

Solution D: 2.96 g DC-193 in 483 g of resin solution

Solution E: 3,26 g Zonyl FSN-100 surfactant (duPont) in 475 g of resinsolution

Solution F: 2.90 g Arlasolve 200 surfactant (ICI) in 470 g of resinsolution

Each of these solutions was reheated to somewhat above 50° C. on a hotplate using a magnetic stirrer in order to re-dissolve any precipitatesthat may have formed. These solutions were poured into separate aluminumfoil lined trays and subjected to drying in a vacuum oven at 140° to150° F. while occasionally breaking up the solids from the uppersurfaces and emptying the cold trap as needed. The respective driedproducts from the solutions were powdered using a household blenderoperated at the highest speed. The powders were sifted through a No. 25sieve and stored in plastic jugs. In the two-stage microwave-thermaloven procedure, the following conditions were used:

Product A: microwave 15 minutes at 1.5 kw; thermal oven 470° F. for 1hour 3 minutes

Product B: microwave 15 minutes at 1.5 kw; thermal oven 480° F. for 1hour

Product C: microwave 10 minutes at 1.5 kw; thermal oven 470° F. for 1hour 6 minutes

Product D: microwave 15 minutes at 1.5 kw; thermal oven 470° F. for 61minutes

Product E: microwave 15 minutes at 1.5 kw; thermal oven 480° F. for 67minutes

Product F: microwave 15 minutes at 1.5 kw; thermal oven 480° F. for 62minutes

In each free-rise foaming was employed.

Results

Product A: foam density was 0.53 pcf; LOI was 35-36

Product B: foam density was 0.61 pcf; LOI was 41-42

Product C: foam density was 0.53 pcf

Product D: foam density was 0.71 pcf

Product E: foam density was 0.78 pcf

Product F: foam density was 0.89 pcf

EXAMPLE 9 Formulation

[(0.72 mol MDA+0.31 mol TGD) per mole BTDA]+2.0% DC-193 based on thecombined weights of the monomers.

Procedure

In this Example, a ten gallon, stainless steel reactor equipped withheating or cooling coils supplied by cold or hot running water, a singleblade impeller and an overhead condenser system was used. The followingingredients were charged into the reactor: BTDA (6.543 kg; 20.00 mols),methanol (5.126 kg; 158.4 mols), and distilled water (258 g). Whilestirring the solution, its temperature was raised from 80° F. to 135° F.over a 40 minute period at which time the heating was discontinued. Thesolution was stirred for another 10 minutes during which its temperaturedecreased to 125° F., yielding a clear amber solution of BTDE inmethanol. The diamines were charged as follows: TGD (1.961 kg; 6.12mols) and MDA (2.825 kg; 14.22 mols) using methanol dilution. Then, 227g of DC-193 was added, again using methanol as a solubility aid. Inthese operations a total of 3.443 kg of methanol was introduced into thereactor. The mixture was then heated and stirred for about 45 minutesuntil the temperature reached 55° C. at which point the heating wasdiscontinued. The reaction solution was stirred for an additional 1 hourand 7 minutes. Portions of the reaction solution were dried in a spraydryer. A representative sample of the resultant polyimide precursor wasfoamed in a mold under the following conditions: microwave: 25 minutesat 2.75 kw; thermal oven: 450° F. for 63 minutes.

Results

The polyimide foam had a tensile strength of 19 psi and a density of1.06 pcf. The roller fatigue test on a specimen of 4.25 inch thicknesswas terminated at 23,373 cycles with no weight loss and no loss in IFDat 40% deflection. The foam sustained a 3% to 4% thickness loss but hadcracks only at the ends of the roller travel.

When producing the polyimides of this invention for applications otherthan foams (e.g., for structural applications, adhesives, films, or thelike), the mixture of diamines may be reacted with the organictetracarboxylic acid or a derivative thereof such as its dianhydride,its acid halides, it salts, or its esters. Use of tetracarboxylic aciddianhydrides is most preferred for these particular reactions because oftheir high reactivity.

It will be understood that various changes and modifications can be madein the details of procedure, formulation and use without departing fromthe spirit and scope of the invention.

What is claimed is:
 1. A polyimide foam prepared by reaction of anorganic tetracarboxylic acid or derivative thereof with (a) about 1 toabout 50 mol percent of a diester of (i) a primary amino-substitutedaromatic carboxylic acid, and (ii) a polymethylene glycol, and (b) atleast one aromatic or heterocyclic primary diamine.
 2. A polyimide foamas defined in claim 1 wherein the polymethylene glycol contains 3 to 8carbon atoms in the molecule.
 3. A polyimide foam as defined in claim 1wherein the reaction is conducted in the presence of a surfactant, andoptionally, a flame retardant.
 4. A polyimide foam as defined in claim 1wherein the foam is prepared from an organic tetracarboxylic acid ester.5. A polyimide foam prepared by reaction of di(lower alkyl) ester ofbenzophenone tetracarboxylic acid with (a) about 20 to about 50 molpercent of diester of (i) a primary amino-substituted benzoic acid and(ii) a polymethylene glycol having 3 to 8 carbon atoms in the molecule,and (b) at least one aromatic or heterocyclic primary diamine.
 6. Apolyimide foam as defined in claim 5 wherein the reaction is conductedin the presence of a silicone surfactant, and optionally, a flameretardant.
 7. A polyimide foam as defined in claim 5 wherein theamino-substituted benzoic acid is p-aminobenzoic acid, the polymethyleneglycol is trimethylene glycol, and the aromatic or heterocyclic primaryamine is 4,4'-methylenedianiline or 4,4'-oxydianiline, or both.
 8. Apolyimide foam as defined in claim 7 wherein the aromatic orheterocyclic diamine further includes 2,6-diaminopyridine.
 9. Apolyimide foam as defined in claim 7 further including anamino-terminated butadiene-acrylonitrile reactant.
 10. A polyimide foamprepared by reaction of an organic tetracarboxylic ester with (a) about1 to about 50 mol percent of a diester of (i) a primaryamino-substituted aromatic carboxylic acid, and (ii) a polymethyleneglycol, and (b) at least one aromatic or heterocyclic primary diamine,in the presence of a surfactant, said foam having a fatigue life asdetermined by ASTM test procedure D 3574-81 on foam specimens in therange of from three to five inches in thickness of at least 15,000cycles, and an indentation force deflection as determined by ASTM testprocedure D 3574-81 on foam specimens of one inch thickness of less than40 pounds of force at 25 percent deflection and less than 180 pounds offorce at 65 percent deflection.
 11. A polyimide foam as defined in claim10 wherein said organic tetracarboxylic ester consists essentially amethyl or ethyl ester of benzophenone tetracarboxylic acid and saidaromatic or heterocyclic primary diamine is composed predominantly of4,4'-methylenedianiline or 4,4'-oxydianiline, or both.
 12. A polyimidefoam as defined in claim 11 wherein said surfactant is a siliconesurfactant.
 13. A polyimide foam as defined in claim 12 wherein thereaction further includes an amino-terminated butadiene-acrylonitrilecopolymer.
 14. A polyimide foam as defined in claim 1 wherein thereaction is conducted in the presence of a blowing agent.
 15. Apolyimide foam as defined in claim 1 wherein the organic tetracarboxylicacid or derivative thereof used in the reaction is a lower alkyl esterof an organic tetracarboxylic acid.
 16. A polyimide foam as defined inclaim 15 wherein said diester is a diester of an amino-substitutedbenzoic acid and a polymethylene glycol having 3 to 8 carbon atoms inthe molecule.
 17. A polyimide foam as defined in claim 15 wherein thereaction is conducted in the presence of at least a surfactant.
 18. Apolyimide foam as defined in claim 15 wherein said aromatic orheterocyclic primary amine is 4,4'-methylenedianiline or4,4'-oxydianiline, or both.
 19. A polyimide foam as defined in claim 18wherein the aromatic or heterocyclic diamine further includes adiaminopyridine.
 20. A polyimide foam as defined in claim 18 wherein thereaction further includes an amino-terminated butadiene-acrylonitrilereactant.
 21. A polyimide foam as defined in claim 15 wherein saiddiester is trimethylene glycol di-p-aminobenzoate.
 22. A polyimide foamas defined in claim 21 wherein said aromatic or heterocyclic primaryamine is 4,4'-methylenedianiline or 4,4'-oxydianiline, or both, andwherein the reaction is conducted in the presence of at least asurfactant.
 23. A polyimide foam as defined in claim 22 wherein thereaction further includes a diaminopyridine or an amino-terminatedbutadiene-acrylonitrile reactant.
 24. A polyimide foam as defined inclaim 15 wherein the lower alkyl ester of an organic tetracarboxylicacid is a lower alkyl ester of benzophenone tetracarboxylic acid.
 25. Apolyimide foam as defined in claim 24 wherein said lower alkyl ester ofbenzophenone tetracarboxylic acid is predominantly the dimethyl ordiethyl ester, wherein said aromatic or heterocyclic primary amine is4,4'-methylenedianiline or 4,4'-oxydianiline, or both, wherein thereaction further includes a diaminopyridine or an amino-terminatedbutadiene-acrylonitrile reactant, and wherein the reaction is conductedin the presence of at least a silicone surfactant.
 26. A polyimide foamas defined in claim 5 wherein the reaction is conducted in the presenceof a surfactant.
 27. A polyimide foam as defined in claim 26 wherein thedi(lower alkyl) ester of benzophenone tetracarboxylic acid ispredominantly a dimethyl or diethyl ester and wherein the aromatic orheterocyclic primary diamine is 4,4'-methylenedianiline or4,4'-oxydianiline, or both, and wherein the reaction further includes adiaminopyridine or an amino-terminated butadiene-acrylonitrile reactant.28. A polyimide foam as defined in claim 10 wherein said organictetracarboxylic ester consists essentially of a lower alkyl ester, andwherein the polymethylene glycol contains 3 to 8 carbon atoms in themolecule.
 29. A polyimide foam as defined in claim 10 wherein theorganic tetracarboxylic ester is predominantly a lower alkyl ester ofbenzophenone tetracarboxylic acid, wherein the polymethylene glycolcontains 3 to 8 carbon atoms in the molecule, wherein the aromatic orheterocyclic primary diamine is 4,4'-methylenedianiline or4,4'-oxydianiline, or both, and wherein the reaction further includes adiaminopyridine or an amino-terminated butadiene-acrylonitrile reactant.